BANGKOK — Three years after bloody street protests left more than 90 dead and thousands injured, Thailand's simmering political tensions are once again threatening to boil over.

Weeks of protests against a deeply unpopular amnesty bill have escalated into an attempt by opposition leaders to overthrow the government of Prime Minister Yingluck Shinawatra and her Pheu Thai Party.

The opposition group, led by former members of the Democrat Party, is calling for a rally of 1 million supporters in Bangkok on Sunday to protest a government they claim is deeply corrupt and still under the control of Yingluck's brother, Thaksin Shinawatra, the deposed former prime minister.

"The Thaksin regime has abused the power that it has borrowed from the people, and it's time that Thaksin should return that power back to the people," said Akanat Promphan, a former member of parliament who is spokesman for the opposition.

The protesters hope to display enough popular support Sunday to convince civil servants, the police and the military to join their cause. "They're the mechanism of the Thaksin regime. Without them, Thaksin and his power cannot function," Akanat said.

"If they can paralyze the country by a general nationwide strike, across the board — civil service, military, police and so on — then the country is ungovernable," said Thitinan Pongsudhirak, director of the Institute of Security and International Studies at Chulalongkorn University. "They would then hope for an intervention from the military or the judiciary."

There is recent history in Thailand of both methods of removing the government. In 2006, a military coup ousted Thaksin, then the prime minister. And in 2008, Thailand's Constitutional Court dissolved the People's Power Party (PPP), composed primarily of Thaksin allies, over charges of electoral fraud.

Adding to the atmosphere of political showdown in Bangkok has been the arrival of members of the United Front for Democracy Against Dictatorship (UDD) — known as the red shirts — a political pressure group tightly allied with Thaksin and the Pheu Thai Party.

In 2010, massive street protests in Bangkok were led by the red shirts against Prime Minister Abhisit Vejjajiva and his Democrat Party government, which had been installed by a judicial coup.

The protesters were demanding that government be dissolved and early elections held. The military stepped in, and their crackdown in April and May 2010 became one of the most violent periods in modern Thai history.

Earlier this week, the red shirts held a two-day rally and announced another rally to begin Sunday at a Bangkok stadium, with no end date specified.

Red shirt leader Tida Tavonseth said the opposition leaders are trying to drag the country back to the days when it was run by a small group of elites and are using the mass protests as cover.

"They are trying to provide legitimacy for independent organizations to take over the government," she said. Tida said she expected the red shirt turnout to eventually reach 100,000 people as more supporters arrive from outside of Bangkok.

This wave of political turmoil started with the blanket amnesty bill that was pushed through the lower house of parliament in October. The bill covered politically related crimes from the 2006 coup through the brutal 2010 crackdown on Thai protesters, but many saw it as a ploy to allow Thaksin to return from self-imposed exile in Dubai. He was convicted of criminal corruption charges in 2008 and has had $1.4 billion in assets seized.

Tens of thousands of protesters took to the streets daily, blowing whistles and calling for the bill to be scrapped. Bowing to public pressure, the Thai Senate voted it down on Nov. 11, but by then political scars had reopened and opponents of Thaksin saw an opportunity to press their cause.

In many ways, Thaksin remains at the eye of the political storm. Protesters, who continue to gather nightly at Bangkok's Democracy Monument, chant derisive slogans and carry signs lampooning the former prime minister.

One opposition supporter, Montida Tiasuwan, said the protesters were there for three reasons: "We love the King, we hate the Thaksin government, and we're against corruption."

Thaksin's base is mainly drawn from the rural, northern parts of the country, where his populist policies are widely supported.

Despite his deep unpopularity in Bangkok, Thaksin's political parties have won every election they've run in since 2001. Bangkok is the traditional seat of Thai power, but the masses of Thaksin supporters from outside the capital have proved that they hold the electoral advantage.

The effects of the latest round of protests are already being felt: The Thai Chamber of Commerce has publicly expressed fears about the economic impact caused by the prolonged protests, and several countries have been prompted to issue travel warnings for Thailand in recent weeks, including the U.K., France, Germany, Australia and Japan. (The United States, however, has not issued a travel alert or travel warning.)

But even if this latest round of protests doesn't end in violence or a coup, the question remains whether Thailand can break free of its cycle of political upheaval.

Thitinan Pongsudhirak of Chulalongkorn University said ultimately the country will need to avoid fighting its political battles on the streets and in the courthouse — or else the cycle will perpetuate.

"We really have to stick to the rules and abide by the institutions — and strengthen these institutions and these rules for the long-term development of Thai democracy," he said. "If we go with street protests and extra-constitutional and extra-parliamentary measures, there is no end to it. The other side can do it, too."